Known in the art is a method for preparing a carbamide-furan resin by way of modification, with furyl alcohol, of a sparingly-soluble in water urea-formaldehyde resin prepared by reaction of urea, monomethylolurea and formaldehyde. The modification process is conducted at a pH value within the range of from 5 to 6.5 and at a temperature of from 95.degree. to 105.degree. C.
The thus-prepared carbamide-furan resin is quite suitable for the manufacture of foundry cores by the "hotbox" method. However, in the production of cores from sand-resin mixtures on the basis of said resin in a cold molding equipment, said cores possess a low rate of cold curing and a tensile strength of cold-cured samples made of said sand-resin mixtures does not exceed 0.55 kgf/cm.sup.2 after 0.5 hour and 2.1 kgf/cm.sup.2 after 4 hours.
Known in the art is a method of preparing a carbamide-furan resin comprising modification, by means of furyl alcohol, of a urea-formaldehyde resin sparingly soluble in water and prepared by reacting urea, formaldehyde and tetraoxane. The modification is conducted at the temperature of 90.degree. C. and pH=5.5.
The resulting carbamide-furyl resin ensures the production, in a high molding equipment, of cores possessing a high mechanical strength. At the same time, the use of this resin for the cold manufacture of cores, the core mixtures have a low curing rate; tensile strength of coldcured samples does not exceed 0.6 kgf/cm.sup.2 after 0.5 hour and 2.05 kgf/cm.sup.2 after 2 hours.
Therefore, the prior art methods of preparing carbamide-furan resins have disadvantages residing in that these resins, while ensuring a high mechanical strength of foundry cores in the "hot-box" process, are much less suitable for the manufacture of foundry cores in a cold moulding equipment due to a long duration required for curing of core mixtures based on said resins and an insufficient mechanical strength of the resulting cores. For this reason, carbamide-furan resins as produced by the prior art methods can be used for the manufacture of foundry cores in a cold moulding equipment only in such processes, where the curing time of cores is not limited and core shape is such that the resulting low mechanical strength of cores is acceptable.